A compass says little at a pole
![[NASA/Marshal Space Flight Center] Similar magnetic fields: Earth and a bar magnet](earth-magnet.jpg) Q: What will a compass do when I am standing on the geomagnetic pole near the North Pole? -- Bert
Meijers, Vaals, the Netherlands
A: Your compass will tilt towards the ground because that's the way that the magnetic field points. A compass
aligns itself with the field flux lines just like iron filings do.
[NASA/Marshal Space Flight Center] Similar magnetic fields: Earth and a bar magnet
Pointing down, of course, doesn't tell you much. Therefore, at the north-seeking pole (about 79° N, 105° W,
near Ellef Ringnes Island, Canada), the compass is useless as a direction pointer because the needle hits the
bottom of the compass.
Our geomagnetic poles have flipped (north becoming south and visa versa) more than 20 times in the past five million years. In fact, the
present south magnetic pole is near the North Pole.
A flow of electric current deep inside Earth maintains the field that has existed for at least three billion years, says Gary Glatzmaier,
professor of earth sciences at the University of California at Santa Cruz. The iron-alloy core of Earth is a solid ball surrounded by a
molten shell. The cooling Earth solidifies some of the molten iron from the alloy-- leaving the lighter constituent of the alloy in molten
form. This change from liquid to solid releases heat. Moreover, the lighter and consequently more buoyant parts of the alloy rise in the
liquid shell.
The combination creates a great roiling movement in the rest of the molten core. Coriolis force, due to the Earth's rotation, twists and
shears the flow into a helical pattern. The motion of an electrically-charged fluid in a magnetic field causes electrons in the liquid iron to
flow, generating an electric current. Earth's core functions, in this fashion, somewhat like an electric generator. The current causes an
electromagnetic field about Earth similar to that of a bar magnet.
The twisting flow pattern of the molten alloy is slightly unstable and sometimes this generates a new magnetic field oriented in the
opposite direction. When this happens, our poles to switch. The last pole flip occurred about 800,000 years ago, when mammoths
roamed the world.
Further Surfing:
Los Alamos National Lab: the why and how of Earth's magnetic field
National Geophysical Data Center: the geomagnetic field
MadSci Network: a compass at Hudson's Bay
(Answered September 6, 2002)
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